Neopterin is a marker of the activation of cell-mediated immunity. The aim was to determine whether the concentrations of neopterin differ in plasma, ascites and ovarian cyst fluid between patients with ovarian cancer and patients with benign ovarian tumours. Neopterin was measured in 29 patients with cystic ovarian tumours of unknown histology. 14 ovarian cancers and 15 benign ovarian tumours were diagnosed histologically. Patient age and tumour size did not differ significantly between the groups. Neopterin levels were determined in plasma, and in ascites and cyst fluid by ELISA. The neopterin concentration in plasma, ascites and ovarian cyst fluid was significantly higher in patients with ovarian cancer compared with patients with benign ovarian tumours of the same size. The study shows that activation of cell-mediated immunity, defined as increased formation of neopterin, was increased in patients with ovarian cancer compared with patients with benign ovarian tumours.